tidb-functions: extend tidb_decode_key docs

[Joyinqin]

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

Extend documentation for TIDB_DECODE_KEY.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

What is the related PR or file link(s)?

• This PR is translated from: tidb-functions: extend tidb_decode_key docs docs#5877
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[Joyinqin]

/label needs-cherry-pick-release-5.0
/label needs-cherry-pick-release-5.1
/translation from-docs

[TomShawn]

LGTM

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Details

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[TomShawn]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Details

Commit hash: 66a0e58

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #6661.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #6662.